1. Field of the Invention
The present invention relates to a rotary-anode type X-ray tube and, more particularly, to an improvement in the structure of a bearing for supporting a rotary-anode of the X-ray tube.
2. Description of the Related Art
As is know, in a rotary-anode type X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft which have a bearing portion therebetween, and an electron beam emitted from a cathode is applied to the anode target while the anode target is being rotated at high speed by energizing an electromagnetic coil arranged outside a vacuum envelope, thereby the target irradiates X-rays. The bearing portion is constituted by a rolling bearing, such as a ball bearing, or a hydro-dynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium--indium--tin (Ga--In--Sn) alloy, which is liquid state during an operation. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-117531, 62-287555, 2-227947, and 2-227948.
In the rotary-anode type X-ray tubes disclosed in the Publication or Disclosures, the gap between bearing surfaces of a hydro-dynamic pressure type sliding bearing is kept at, for example, 20 .mu.m and filled with liquid metal lubricant. If air is removed from the gap while the X-ray tube is being assembled, or gas is produced in the lubricant when the X-ray tube is energized, the gap is locally free from liquid metal lubricant due to the bubbles of air or gas. Otherwise, the lubricant may leak from the bearing, together with the bubbles. Accordingly, if the air or gas is removed from or introduced into the sliding bearing, the bearing cannot stably operate for a long period of time. If the lubricant leaks from the bearing into the vacuum envelope of the tube, the high voltage characteristic of the X-ray tube may be degraded.